Currently, as non-aqueous electrolytic secondary batteries for use in portable apparatuses such as mobile phones, lithium ion secondary batteries have been commercialized. In these secondary batteries, there are used a lithium-transition metal complex oxide material such as lithium cobalt oxide as a positive electrode; a graphite material or a carbonaceous material as a negative electrode; a solution obtained by dissolving a lithium salt in an organic solvent as a non-aqueous electrolytic solution; and a porous film as a separator.
In recent years, as the portable apparatuses become thinner, it is desired that the batteries have a reduced thickness. For this purpose, the thickness of a casing for containing therein the positive electrode, negative electrode, separator and non-aqueous electrolytic solution must be reduced.
When the thickness of the casing for the battery is reduced, a phenomenon such as a gas generation in the battery or an elevation of the battery internal temperature directly causes an accident such as deformation of the battery, gas leakage or ignition. Therefore, for securing the safety of the battery, it is necessary to suppress the gas generation in the battery or the elevation of the battery internal temperature in extremely low level.
For meeting the demand, Japanese Prov. Patent Publication No. 235868/2000 proposes a non-aqueous electrolytic secondary battery which comprises a casing comprised of a sheet comprising a resin layer and having a thickness of 0.5 mm or less, and an electrolytic solution comprised of a non-aqueous solvent comprising more than 50 to 95% by volume of γ-butyrolactone, based on the total volume of the non-aqueous solvent. It is stated that the non-aqueous electrolytic secondary battery which comprises the non-aqueous electrolytic solution comprised of the non-aqueous solvent comprising more than 50 to 95% by volume of γ-butyrolactone based on the total volume of the non-aqueous solvent, and further comprising ethylene carbonate and vinylene carbonate is especially preferable.
When using the electrolytic solution disclosed in the above patent document, an effect of improving the discharge characteristics at a large current or the cycle characteristics can be obtained; however, inflation of the casing due to gas generation in the battery when stored at high temperatures is not satisfactorily prevented.
In addition, Japanese Prov. Patent Publication No. 40526/2000 proposes a non-aqueous electrolytic solution for secondary battery having excellent charge-discharge efficiency, which solution comprises a non-aqueous solvent comprising a cyclic carbonate such as a vinylethylene carbonate derivative, and an electrolyte. However, when using an electrolytic solution comprising γ-butyrolactone, the cycle characteristics are not satisfactory.
The present invention has been made with a view toward solving the above-mentioned problems, and an object of the present invention is to provide a non-aqueous electrolytic secondary battery which is advantageous in that gas generation in the battery when stored at high temperatures is suppressed to prevent the casing of the battery from inflating, but also in that both the discharge characteristics at a large current and the charge-discharge cycle characteristics are improved.